1. Field of the Invention
The present invention relates to a process for fractionating cellulosic fibers. The cellulosic fibers prepared from such a process may be used to prepare a handsheet or other structure that may be used in a disposable absorbent product intended for the absorption of fluids such as body fluids. Other possible applications of these fibers include various disposable paper products such as tissue and towel.
2. Description of the Related Art
Cellulosic fibers are well known and are used in a wide variety of applications. However, natural or generally untreated cellulosic fibers have been found to generally not provide a level of performance that is desired in certain applications such as the absorption or handling of fluids such as body fluids. As such, it is often desirable to increase the liquid absorbent capacity or the resiliency of the cellulosic fibers being used in such applications. Typically, this has meant that the cellulosic fibers being used have been structurally modified in order to improve the performance of such modified fibers in a particular application.
One known method for modifying cellulosic fibers is to chemically crosslink the cellulosic fibers. In general, a chemical crosslinking agent is added to either a solution containing cellulosic fibers or to swollen cellulosic fibers. The chemical crosslinking agent is then allowed to form crosslinks either within an individual cellulosic fiber or between separate cellulosic fibers. Such processes inherently result in the use of a separate crosslinking agent thereby increasing the costs of manufacturing the chemically crosslinked cellulosic fibers. Additionally, the use of certain crosslinking agents typically requires specialized handling procedures, further increasing the costs of manufacturing, and potentially limiting the applications for which the chemically crosslinked cellulosic fibers may be used. Another disadvantage concerning the use of chemical crosslinking agents is that they are often based on chemicals such as aldehydes which exhibit certain degrees of toxicity.
A variety of chemical treatments of cellulosic fibers are also known. An example of a well known chemical treatment of cellulosic fibers is a mercerization process wherein cellulosic fibers are treated with, typically, sodium hydroxide under suitable conditions to convert the cellulose from its native form into a more thermodynamically stable, less crystalline form. Because the mercerized cellulose is less crystalline and more amorphous, the mercerized cellulose is generally more accessible for further treatment with additional reagents.
These and other known processes for chemically treating cellulosic fibers typically disperse the cellulosic fibers in a solvent, such as an aqueous solution. However, it has been generally recognized that in order to ensure proper mixing of the cellulosic fibers and whatever chemicals are being used to treat the cellulosic fibers as well as to ease the bulk transport of the treatment mixture, such known processes must have the cellulosic fibers present in the solvent at a low consistency. Such processes therefore generally result in the use of more of the solvent in which the chemical treatment takes place, or the chemical treatment agent being used, than would ideally be needed, thereby increasing the costs of manufacturing the chemically treated cellulosic fibers. Additionally, the use of sodium hydroxide, or other caustic agents, typically requires specialized handling procedures as well as recycling processes to ensure that such materials are not discharged to the environment.
Another known method for modifying cellulosic fibers is to mechanically treat the cellulosic fibers. One example of such a mechanical treatment process is wherein the cellulosic fibers are subjected to a high shear force which generally results in highly twisted or curled cellulosic fibers. However, such mechanical treatment processes generally require the use of specialized equipment and the use of large amounts of energy, thereby increasing the costs of manufacturing the mechanically treated cellulosic fibers. Besides, without any additional treatment, the fibers modified only by mechanical treatment generally do not preserve their curl in wet conditions because they swell and collapse. Therefore, mechanical modification is generally not quite sufficient for cellulosic fibers which are used for absorbent structures requiring more porosity or capacity.
It is therefore an object of the present invention to provide a process for the preparation of cellulosic fibers in which the amount of solvent and chemical treatment agents used in the process is minimized or completely eliminated.
It is also an object of the present invention to provide a process for the preparation of cellulosic fibers which will significantly reduce the costs of manufacturing.
It is also an object of the present invention to prepare fractionated cellulosic fibers that exhibit improved liquid handling properties as compared to unfractionated cellulosic fibers.
It is also an object of the present invention to prepare fractionated cellulosic fibers that are useful in preparing paper filter structures.